There seems to be two faults discussed here - (1) noise and (2) Combined Noise and overheating. I have (2) noise/overheating version. A big concern, as it’s likely to be an overloaded power supply, that could fail. My Mac is connected to a second monitor, but that should not contribute to the problem, as it only uses signal, not power. Since aiming a desk fan at the iMac vent, with a cardboard funnel focussing the air at the vent, the noise and heat went away. I already had a UPS / line filter in place and that made no difference.
LEDs can be very noisy too and interfere with TV reception. There is no easy way to find out which LEDs are noisy, so just purchase one and try it out. If your iMac is getting very hot, the noise can be the power supply or other section getting very stressed. I run a desk fan on my iMac every warm day and the arcing noise disappeared. I don't like to increase internal fan speeds, as it shortens the life of the tiny fans. Look up advice on how to clean an iMac too. Thats my next step, along with a tube from the desk fan to the vent. When trying to have an elegant look, Apple ensured overheating problems were common and made it near impossible to dismantle to clean and fix. I don't care about the elegant slim look, I'd much prefer reliability and ease of service. I previously did quite a few things inside my Bondi Blue Mac - which still runs fine